Types of DC Machines its specification and Applications

D.C. MACHINES

DC machines are electromagnetic device having  one or more than one coil, which runs from a DC source and generate mechanical power or convert mechanical power into Electrical power.

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       DC machines operates on the principle of electromagnetic induction, whenever a conductor cuts magnetic lines of force & a voltage is induced in the conductor.Moving of a conductor across a magnet produces an electric energy called EMF.

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What things to be need for making 12v DC motor?

Types of DC machine:

1) D.C Generator

2) D.C Motor

Construction of DC machine:

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Magnetic Frame or Yoke:-The outer body of machines is known as yoke.The main functions of yoke are:-

i) Provide mechanical support and acts as a protecting cover for the whole machine

ii)Carries magnetic flux produced by the poles.

To keep the weight low in small DC machines, yoke is made up of cast iron. In large DC machines cast steel or rolled steel is employed.

Pole Cores and Pole Shoes

• The pole cores and pole shoes play an important role for field winding to create  electromagnet.
• Job of a Pole Core is to carry Field windings.
• Field windings are responsible of producing flux.
• It directs the flux from air gap->Armature->To next pole. 
• Pole shoe spread out the flux in the air gap and also being a large cross section area, it reduces reluctance of magnetic path.
• It also support the field coils Field Coils or windings
• The field coils consist of copper wires.
• The field windings are nothing but coil/wire which carries current.
• This current is responsible for the production of flux.
• These wound coils are placed on the pole cores.
• The field coils are excited by the DC voltage which creates electromagnets in DC machines

Armature Core:-The slots carries armature conductor.The armature core is used for following purposes.

• It provides housing to armature conductors and causes them to rotate
• In large machines the air ducts/holes are provided in core for cooling.
• Key is used to connect shaft.
• To avoid eddy current losses armature is made up of thin laminations(Cast iron/steel).

Armature Windings:The armature coils are usually former wound.First rectangular coils are formed and then suitably shaped in a coil puller.

• The conductors of a coil are insulated from each other.
• These conductors are placed in the slots of armature core by providing suitable insulation between the slot and conductor

The armature winding are of two types:

1) Lap Winding: The number of parallel paths is exactly equal to the number of poles(P).

2) Wave Winding: Number of parallel paths(A)=2

Commutator:-It is a mechanical rectifier It is made up of segments..

• Segments are made up of copper and it is separated by mica.

• The function of the commutator is to collect current from armature conductors.
• It acts like a rectifier which converts alternating current into DC. (unidirectional current)
• It is cylindrical shaped built up wedge-shaped segments of high conductivity hard drawn or drop forged copper.

Brushes and Bearings

• These are stationary.

• The brushes are used to collect or provide current to commutator.

• These are made up of graphite or carbon usually rectangular shaped block.

• These brushes are housed in brush holders

• The brush holder is mounted on a spindle and brushes can slide in

the rectangular box open at both ends

• Ball bearings are provided for small machines and roller bearing are preferable in large machines.

Basics of DC Generator

• Direct current machines are energy transfer devices.Which can be function as either a motor or a generator.

• DC motors and generators have an equivalent basic construction, differing primarily within the energy conversion.

• In a DC generator, Mechanical Energy is input and output is Electrical Energy

• In a DC motor, Electrical Energy is input and output is Mechanical Energy

• To induce a voltage into a conductor, there should be Relative motion between the magnetic field and conductor.

• A DC generator provides these three conditions to supply a DC voltage output.

• A basic DC generator has four basic parts: (1) a magnetic field (2) one conductor, or loop (3) a commutator and (4) brushes

• one conductor, shaped within the sort of a loop, is positioned between the magnetic poles.

• As long because the loop is stationary, the magnetic flux has no effect (no relative motion).

• If we rotate the loop, the loop cuts through the magnetic field, and an EMF (voltage) is induced into the loop as per Faraday’s Law of Electromagnetic Induction.

• once we have relative motion between a magnetic flux and a conductor ,an EMF is induced into the conductor.

• The magnitude of the induced EMF depends on the field strength and the rate at which the flux lines are cut i.e. speed. The stronger the sector or the more flux lines cut for a given period of your time , the larger the

•  The direction of the induced current flow are often determined using the “Fleming’s left-hand rule" for generators.
•  This rule states that if you point the index of your left within the direction of the magnetic flux (from North to South) and point the thumb in the direction of motion of the conductor, the center finger will point within the direction of current flow .
• within the generator shown in Fig, for instance , the conductor closest to the N pole is traveling upward across the field; therefore, the present flow is to the proper , lower corner. Applying the left-hand rule to each side of the loop will show that current flows during a counter-clockwise direction within the loop.
• The commutator is play a role as a mechanical rectifier which converts the AC voltage generated  into a DC voltage.
• It also is a way of connecting the brushes to the rotating loop.
• The aim of the brushes is to attach the generated voltage to an external circuit. In order to try to this, each brush must make contact with one among the ends of the loop.
•  Since the loop or armature rotates, an immediate connection is impractical.
• The brushes, usually made from carbon , rest against the commutator and slides along one half of the commutator and then along the other half.
• whenever the ends of the loop reverse polarity, the brushes switch from one commutator segment to subsequent . This means that one brush is usually positive with reference to another.
• The voltage between the brushes fluctuates in amplitude,between zero and some maximum value, but is always of the same polarity 
• In this way commutation is accomplished in a DC generator.
• One important point to notice is that, because the brushes pass from one segment to the opposite , there's a moment when the brushes contact both segments at an equivalent time. The induced voltage at this point is zero.
• If the induced voltage at now weren't zero, extremely high currents would be produced thanks to the brushes shorting the ends of the loop together.
• the purpose at which the brushes contact both commutator segments, when the induced voltage is zero, is named the "neutral plane”.

EMF equation of DC generator 

EMF equation of DC Generator

let Z conductors are divided into A paths.

Therefore emf induced in the total armature windings:

E=

Construction and working of DC Motor

What is Back e.m.f ?

When armature rotates, the conductor also rotates and hence cut the flux, Emf is induced,due to this emf, current starts flowing within the conductor.

This currents opposes Ia which is flowing because of the main supply.

Direction of back emf (Eb) is opposite to supply voltage , hence we call this emf as a back emf.

Relation between back emf and supply vtg:            Eb=Vt-IaRa

Where, Eb=back emf, Vt=terminal voltage

Ia=armature current,Ra=armature resistance

Advantages of Back emf:

Energy conversion is possible due to back emf.

The back emf makes the motor self regulating so that it draws as much current as necessary.

Types of DC motor in specification and Applications-

Types of D.C Motor & its Characteristics:There are mainly three types of DC Motor

1. Series Motor

2. Shunt Motor and 

3. Compound Motors.

1.Series Motor:

A motor is said to be DC Series motor if field winding is connected in series with the armature winding.This motor develops a large amount of starting torque. 

D.C Motor Characteristics

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